The field of this invention is that of motor protectors and the invention relates more particularly to a refrigerator compressor motor system having inherent motor overload protection and to motor protectors for use in such a system.
Thermostat metal motor protectors adapted to provide what is called inherent motor overload protection are well known. Such protectors are both current and temperature responsive and provide both short time (locked rotor) and ultimate trip (running overload) protection to prevent overheating of motors due either to large, sharp motor overloads of brief duration or to smaller motor overloads of longer duration. In such an inherent motor protector, a dished thermostat metal means is arranged to have selected thermal coupling to an electrical motor when the protector is mounted in its intended position on the motor. The protector also has an electrical resistance heater system which is connected in series with the motor to carry the motor current for heating the thermostat metal means in response to current flowing in the heater system. During normal motor operation as motor current varies within an anticipated normal range, the combined heating effect from thermal coupling to the motor and from the electrical heater system is such that the thermostat metal means remains unactuated to maintain normal motor operation. However, if a fault condition occurs which would result in overheating of the motor so that motor damage could occur, those combined heating effects heat the the thermostat metal means to a selected actuating temperature so it moves to an inverted dished configuration with snap action to separate protector contacts and interrupt operation of the motor. If the fault condition is due to a small overload or the like and causes a small increase in motor temperature of substantial duration such as could tend to cause eventual deterioration of motor insulation or the like, the ultimate trip characteristic of the protector governs and the protector is typically actuated in response to the combined heating effect of heat transfer from the motor and from a small overload current in the resistance heater system of the protector, thereby to provide what is called running overload protection for the motor. On the other hand, if a fault condition such as a locked rotor occurs, this results in a large, sharp increase in motor current such as would tend to cause a rapid rise in the motor temperature. In this case the short time trip characteristic of the protector governs and the protector is actuated primarily in response to the increase in motor current in the resistance heater system of the protector to interrupt motor operation before the anticipated overheating of the motor occurs, thereby to provide what is called locked rotor protection for the motor. In the typical inherent motor protector, the thermostat metal means subsequently cools to a relatively lower, reset temperature and returns with snap action to its original dished configuration so that, if the fault condition has been corrected during the off-time provided by the protector, normal running operation of the motor is resumed. However, if the fault condition persists, the protector cycles on and off in the manner described for a sufficient period of time without damage to the motor to permit operator intervention to correct the fault condition. For that purpose, the thermostat metal means used in the protector has desirably had a relatively low reset temperature which was selected to provide an off-time characteristic allowing a period for operator intervention which is consistent with the practical cycle life of electrical contacts and other components in the protector.
As will be understood motor temperature occurring during overheating could exceed the temperature limits of insulation materials used in the motor windings. Protector cycling on and off during the continuation of a fault condition in the motor could exceed the cycle life of electrical contacts or other components used in the protectors. Accordingly specifications for motors and motor protectors are typically prescribed in codes established by testing services and industry associations and by governmental bodies and the like in different countries to assure that the motors and protectors have the properties necessary to meet the requirements of various applications. While different codes establish specifications in different terms, the specifications are typically intended to meet related requirements and therefore tend to have similar features. That is, in successfully applying motor protectors to provide inherent overload protections for specific electrical motors to meet those code requirements, the protectors are usually provided with a selected combination of short time (locked rotor) trip and ultimate (running overload) trip characteristics to achieve the desired protection. In that regard, the desired performance characteristics for inherent motor protectors of a particular manufacturer or group of suppliers may be defined by reference to the short time (locked rotor) trip current necessary for tripping the motor protector within a specified short trip time and by reference to the ultimate trip (running overload) current for tripping the protector assuming the current and heat transfer to the thermostat metal means are stabilized at a selected, constant level. For example, in one widely used motor protector specification, the inherent characteristics of a group of motor protectors available for use in a particular category of commercial applications are defined by reference to the short time trip current for a short trip time of ten (10) seconds and to the stabilized ultimate trip current (usually intervals of about 15 minutes) where the effective protector ambient (the ambient determined for the thermostat metal means during normal full load running operation of the motor) is taken to be 65.degree. C., those characteristics typically being referenced in more general terms by expressing the characteristic as a ratio of such short time trip and ultimate trip currents. Inherent motor protectors having performance characteristics defined generally within particular ranges in this manner are then applied to specific motors with respect to the rate of temperature rise and the maximum permitted temperatures of the motor windings and the like by the use of bench tests, thereby to selectively match individual protectors to the motors to meet the code requirements for those motors and for providing the desired inherent overload protection for individual motors likely to be encountered.
In order to achieve present day performance requirements for inherent motor protection as above described, the thermostat metal means in the protector has been provided with selected electrical resistance properties and has been incorporated in the motor circuit as part of the resistance heater system of the protector. That is, in providing sufficient heating for the thermostat metal means to actuate the protector under each of the various different motor overload conditions likely to be encountered, it has been found that, because of heat transfer effects, the heat generated directly in a thermostat metal means having selected electrical resistance properties is more promptly effective than heat transferred from the motor or from other resistance heater means for raising the temperature of the thermostat metal means. Further, the heating effect of the resistance in the thermostat metal means has typically been needed for meeting the complex heating patterns required to provide the desired range of motor protection. As a result, the thermostat metal means has had to be connected in the motor circuit and the need for making electrical connection to the thermostat metal actuating means by electrical contacts or supports or the like has meant that the thermal response characteristics intially provided in the dished thermostat metal means had tended to be altered during protector assembly. This in turn has meant that calibration of the protector has usually been required after protector assembly for meeting motor protection requirements. Accordingly such motor protectors have typically had relatively complex and expensive structures and have usually required complex manufacturing and calibration processes. It would be desirable if the advantages of an inherent motor overload protector could be achieved utilizing a relatively less complex and less expensive motor protector device and a more convenient device assembly procedure while still meeting the demanding requirements of today's industry.